1. Field of the Invention
This invention relates to a method and apparatus for sampling a gas from a stream of products of combustion and, more particularly, for removing water from a gas sample.
2. Description of the Prior Art
The combustion of a fuel and an oxidizer produces products of combustion which include constituents such as NO.sub.2 and SO.sub.2. Moreover, H.sub.2 O in the form of water vapor is also produced during combustion. It is desirable when analyzing emissions of a combustion process to measure from a sample of the products of combustion one or more of the constituents. However, the water vapor condensing in the sample of the products of combustion absorbs some of the constituents to be detected, thereby affecting the accuracy of measurement.
To reduce the absorption of constituents by the water vapor condensing in the sample of the products of combustion, prior art combustion analyzers direct the heated sample of the products of combustion across a "cold plate" positioned between a stream of products of combustion flowing in an exhaust stack and one or more electrochemical sensors, each adapted to detect one or more constituents in the sample of the products of combustion. In theory, when the sample of the products of combustion contacts or passes adjacent the cold plate, the water vapor contained in the sample condenses onto the cold plate. The condensed water vapor then flows to a water trap remote from the cold plate. In practice, however, the cold plate does not precipitate water vapor from the sample of the products of combustion at a sufficient rate to avoid some interference with the measurement of one or more constituents therein. More specifically, the cold plate is formed from a Peltier junction. When exposed to the heated sample of products of combustion, the Peltier junction increases in temperature thereby reducing the ability of the cold plate to precipitate water vapor from the sample of products of combustion. Moreover, Peltier junctions are inefficient, thus requiring a large power supply to provide electrical power thereto.
It is, therefore, an object of the present invention to provide a method and apparatus for extracting water vapor from the sample of the products of combustion more thoroughly than prior art gas sampling systems. It is an object of the present invention to provide to an electrochemical sensor of a combustion analyzer the sample of the products of combustion, having the water vapor removed therefrom, at a temperature within a desired range of operating temperatures for the electrochemical sensor. Still further objects will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.